Improved Current Reference Calculation for MMCs Internal Energy Balancing Control

Abstract

The paper addresses an improved inner current reference calculation to be employed in the control of modular multilevel converters operating during either balanced or unbalanced conditions. The suggested reference calculation is derived based on the AC and DC additive and differential voltage components applied to the upper and lower arms of the converter. In addition, the impacts caused not only by the AC network's impedances but also by the MMC's arm impedances are also considered during the derivation of the AC additive current reference expressions. Another issue discussed in this article regards that singular voltage conditions, where the positive-sequence component is equal to the negative one, may occur not only in the AC network but also internally (within the converter's applied voltages). Several different inner current reference calculation methods are compared and their applicability during the former fault conditions is analyzed. The paper presents a detailed formulation of the inner current reference calculation and applies it to different unbalanced AC grid faults where it is shown that the presented approach can be potentially used to maintain the internal energy of the converter balanced during normal and fault conditions.

Real-Time Optimization-Based Reference Calculation Integrated Control for MMCs Considering Converter Limitations

“…In this section, the proposed optimization-based reference calculation integrated control is compared with two differ-ent MMC control strategies. The first control strategy for comparison is the CCSC (C1) [6], whereas the second one is an energy-based control (C2) [34]. The main difference between strategy C2 and the proposed method is how the controller references are calculated, as they both have the same controllers with the same gains.…”

“…The MMC's circulating currents contain both AC and DC components, which are employed to regulate the internal energy balance of the converter. Such inner current set-points are first calculated through the method described in [34] without considering the limitations of the converter. Then, these values are used as the steady-state trajectories of the online optimization algorithm.…”

“…a) Fault is applied to the system and, b) Fault event is cleared during a SLG fault the strategy C1 [6] is unable to properly regulate the converter; thus, such approach is not included in this case study. Although strategy C2 [34] is capable of maintaining the converter operable during the fault, its formulation does not allow to saturate individual arm currents. This happens due to the modelling formulation which uses differential and additive current components.…”

“…Contrarily, as the optimization problem is formulated considering the currents flowing through each individual arm, the proposed method is capable of specifying maximum allowed values for the currents circulating through the converter's arms. For comparison purposes, the saturations employed in the control strategy [34] have been modified. Now, the maximum allowed three-phase DC and AC additive currents are set to 0.95 pu.…”

“…During normal operation conditions, the upper arm currents present balanced profile and have lower maximum magnitudes compared to the lower arms. Such fact arises due two conditions; firstly, the optimization must maintain the power set-points imposed by the operator; secondly, the formulation of the optimization problem, which avoids zero sequence AC currents to flow into the DC side of the converter (observed when strategy C2 [34] was employed).…”